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844

Appendix II.

We next apply the elementary case to the model H. The
spring b becomes the packing q q, the rubber bar the bolt r; and
the two models c and H are therefore under similar conditions,
only that q q are in compression instead of tension. Applying
the same numbers, the screwing stress is 10 Ibs., felt equally on
r and ^, and shewn on dial /, and a force of 5 Ibs. being exerted
at u, there is a tensile stress of 14 Ibs. in r^ while the compressive
stress in q is reduced to 9 Ibs. Thus, any real conditions may
be ascertained by a diagram such as D, if the resiliences of bolt
and packing be known. If the packing be practically rigid, we
approach the case A, but the flanges always have a certain
elasticity.

In practice, the real difficulty is to find what stress the work-
man will cause in screwing up; hence the rules on p. 402 are
usually adopted, where the screwing stress is made a ratio of
the pressure, and the latter taken as the only guide in calculation.
Also the pressed area is measured to the inner edge of the bolts.

JP. 422. Shaft Couplings.—Mr. Archibald Sharp's coupling,
Fig. 808, is a combination of box and flange, and is an undoubted

X-A/o. or BOLTS

improvement on the latter as regards strength distribution. The
bolts receive shear stress along planes shewn by dotted lines, and
the twisting effort in A is transmitted through surfaces ab, be.
Similarly, B receives the twist through surfaces fb> be The
usual flange-coupling bolt has a shear stress over the whole cross
section, as t b c, but here ring c binds the outer halves of every
bolt, passing the strength ab to &£ %and the bolts are only